This patent document relates to optical communications.
Optical communications use an optical modulator to modulate an optical carrier beam to carry digital bits for transmission over an optical link. An optical communication system can use optical wavelength division multiplexing (WDM) to transmit multiple optical carriers modulated to carry different optical data channels through a single fiber. The performance of optical transmission can be characterized by various parameters, such as the optical signal to noise ratio (OSNR), the data bit error rate (BER) and the data bit rate per wavelength or data spectral efficiency. The signal quality of an optical WDM signal may be degraded by various effects in the optical transmission such as optical attenuation effects in fiber and optical dispersion effects including chromatic dispersion (CD), polarization mode dispersion (PMD) and polarization dependent loss (PDL) in fiber. Various signal modulation techniques and associated signal detection techniques can be used to generate modulated data formats that tolerate signal degradation effects in optical transmission and thus improve the transmission performance.